FIG. 13 shows a conventional X-ray imaging device 101 suitable for X-ray photography in the medical diagnostic field. The X-ray imaging device 101 includes an X-ray conversion layer 102 made of an X-ray converter material including amorphous selenium (a-Se) and a thin-film transistor (TFT) array 103. An incident X-ray is converted into electric charges in the X-ray conversion layer 102, and the electric charges are read by the TFT array 103 to provide an X-ray image. Such devices as the X-ray imaging device 101 are disclosed in JP2001-148475 A and “Development of Flat Panel X-Ray Image Sensor” (Yoshihiro Izumi et al, Sharp Technical Report No. 80, P25-P30, August 2001, Sharp Co., Ltd.).
The detection efficiency or sensitivity of an X-ray image in the X-ray imaging device depends on the amount of charges produced by converting an X-ray in the X-ray conversion layer. In the conventional X-ray imaging devices including the one shown in FIG. 13, the amount of energy detectable as an image is only about 70% at the maximum of the total X-ray energy incident into the surface of the X-ray imaging devices.
The X-ray photography in the medical diagnosis field is strongly required to reduce the exposure of a subject. Therefore, it is most important for the X-ray imaging devices applied to the medical diagnosis field to find the solution to a problem of how to enhance the sensitivity in order to reduce the exposure of a subject.